The methods of arresting fractures in high pressure pneumatic pipelines have been the subject of numerous patents. Typical of such types of patents are U.S. Pat. Nos. 3,096,105; 3,349,807; 3,698,747; 3,768,269; 3,860,039; 3,870,350; 3,990,478; 4,144,125; and 4,148,127.
The control of ductile fractures in pipelines in pneumatic service represents a serious problem. This is also true in those pipelines being used to transport fluids which have a significant vapor phase or which contain significant amounts of dissolved gases. The control of fracture propagation through mechanical reinforcements have been described in many of the above-identified prior art patents. Essentially, fractures propagate in the pipeline due to the gas dynamic forces on the pipe which produce stress at the top of the progressing crack which exceed the ultimate strength of the material at that point. By simply strengthening the material at a given point such that the stresses are reduced well below the material ultimate strength, the crack is prevented from progressing. As shown in certain of the patents, such a reinforcement usually takes the form of a band of multiple bands or rope around the circumference of the pipe.
It has been found that a structural independent reinforcing member is preferable. That is, a reinforcement that is either an integral part of the pipe (thick section reinforcement) or a welded-on-member is not as desirable as a structurally independent member as a consequence of stress concentration occurring at the crack tip. An integral member is subjected to crack stress intensification at the crack tip with the crack being able to progress through the reinforcement. When the reinforcing member is not integral, the stress is more uniformly distributed through the reinforcement and greater forces are able to be resisted. This situation is depicted in FIGS. 1 and 2 of the patent drawings which illustrate welded and non-welded sleeves respectively.
FIG. 1 of the patent drawings illustrates the manner in which the weld in a welded-on-seeve arrestor serves as a communicative path for the crack to initiate in the sleeve. The resultant stress intensification at the crack tip greatly reduces the effectiveness of the additional material of the sleeve. In contrast, the crack passes under the sleeve in the non-welded situation as illustrated in FIG. 2 of the patent drawings. The sleeve in FIG. 2 is stretched as shown by the arrows with the resultant stresses uniformly distributed in the body of the sleeve.
While the loose sleeve such as illustrated in FIG. 2 has certain performance advantages, the fact that the gap between the pipeline and the sleeve is in open communication with the environment poses problems with structural integrity of both the pipeline and the arrestor. Crevice corrosion must be avoided at all cost and the cost and construction problems associated with providing such protection are substantial negative features of this particular approach to fracture control. For example, the existence of a sleeve such as illustrated in FIG. 2 poses unresolved difficulties for over-the-ditch pipe coating machines. For certain sleeve designs, the pipe coating machine must be removed from the pipeline, reinstalled, recalibrated, etc. For closely spaced arrestors, the problems enumerated above would preclude over-the-ditch line coating which is normally the least expensive method of pipe coating.
Other types of crack or fracture arrestors are shown in the prior art but the arrestors substantially reduce the interior diameters of the pipeline which serve as an obstruction to pipeline pigs and an obstruction to unimpeded flow of the gases therethrough which creates pressure differentials or drops.
Therefore, it is a principal object of the invention to provide an improved fracture arrestor.
A further object of the invention is to provide a crack arrestor for pipelines which permits conventional pipeline pigs to pass therethrough.
A further object of the invention is to provide a crack arrestor for pipelines wherein the crack arrestor has a diameter substantially the same as the pipeline to permit conventional over-the-ditch coating machines to be used on the pipeline.
A further object of the invention is to provide a crack arrestor including means for preventing crevice corrosion.
A still further object of the invention is to provide a crack arrestor for a pipeline which may be easily installed in conventional pipelines.
A still further object of the invention is to provide a crack arrestor which is economical of manufacture.
These and other objects will be apparent to those skilled in the art.